Various methods are known in which the piezo actuator is used as a sensor element in order to determine the valve opening instant of a piezo servo-driven injector or in order to measure the idle travel that must be taken into consideration in order to implement precise control of the injection quantity.
In these methods, the capacitance curve of the piezo actuator is analyzed, wherein the valve opening instant is related to the maximum of the capacitance curve. Said method, however, does not deliver precise results in all situations under all conditions.
In the case of a standard piezo servo-driven injector, a valve is actuated counter to the prevailing hydraulic pressure. For safety reasons, the piezo actuator is installed with a predefined play, which yields a predefined idle travel. Said idle travel is dependent on various parameters such as for example the piezo polarization, the actuator temperature, the number of injection processes, the engine speed, but in any case also on the wear of the various valve parts. Owing to these parameters, it is difficult to keep the idle travel in a small tolerance range under all operating conditions. Since the idle travel is of significant importance with regard to the injector function, it is important to measure said idle travel and compensate its effect. For this purpose, the above-described measurement of the valve opening instant is of importance.
For the idle travel measurement, an indirect measurement concept exists. Here, the piezo actuator is actuated and the leakage generated by the valve is measured. Owing to the large dynamic variation of the idle travel, it is of crucial importance here for the idle travel to be measured as often as possible. This however leads to undesired leakage.